This invention relates to a decoder with interpolation of a frame or a field which is skipped or omitted in a moving picture signal transmitted from an encoder to the decoder.
The moving picture signal is typically the picture signal of a television signal. The moving picture signal will therefore be referred to as a television signal in the following description. The television signal represents a sequence of frames and/or fields. As the case may be, either of the frames and the fields will be called an instantaneous picture.
On transmitting a television signal to a receiver, which is typically used in a satellite station of a television broadcast network, at least one instantaneous picture is skipped between a preceding and a succeeding or subsequent instantaneous picture in order to reduce the redundancy in the information transmitted by the television signal from a transmitter to the receiver. Each skipped instantaneous picture is interpolated in the receiver by the use of either or both of signal parts representative of the preceding and the succeeding instantaneous pictures. Such skip of at least one instantaneous picture between the preceding and the succeeding instantaneous pictures is desirous also in a picture recording and reproducing system in reducing the memory capacity of a memory for the television signal.
It is to be noted in connection with the preceding and the succeeding instantaneous pictures that the succeeding instantaneous picture for a skipped instantaneous picture serves as the preceding instantaneous picture for another skipped instantaneous picture which is spaced by the succeeding instantaneous picture for the first-mentioned skipped instantaneous picture, namely, by the preceding instantaneous picture for the last-mentioned skipped instantaneous picture. It is believed that no confusion will arise even the expression "an instantaneous picture" is used in place of the words "a signal part." Furthermore, it will be assumed merely for brevity of the description that the frames are alternatingly skipped in the television signal.
It is already known to use movement vectors in representing movement of a moving or movable body from each instantaneous picture to a next succeeding instantaneous picture. The movement vectors are useful in interpolating each skipped instantaneous picture as an interpolated instantaneous picture. The moving body may be still or stationary in some consecutive instantaneous pictures. In this event, each movement vector becomes a zero vector. Each instantaneous picture may show a plurality of moving bodies. Inasmuch as frame skipping is assumed, the instantaneous pictures will hereafter be called frames.
On resorting to the movement vectors, each frame is either one-dimensionally or two-dimensionally divided into a predetermined number of picture blocks of a common area so that the picture blocks of a frame be in one-to-one correspondence to those in another frame. Each picture block usually consists of a plurality of picture elements. The movement vectors are detected or calculated for the respective picture blocks of each frame as will later be described in detail.
For the time being, a definition of each movement vector will be given as follows. Let a part of a moving body be in a particular one of the picture blocks of a first frame and the part be in a specific one of the picture blocks of a second frame which is next subsequent to the first frame. The movement vector for one of the particular and the specific picture blocks may be a radius vector having a starting point at the particular picture block and an end or terminal point at a picture block which corresponds in the first frame to the specific picture block.
In a decoder of the receiver, movement of a moving body in each skipped frame is interpolated by using the movement vectors of one or both of the preceding and the succeeding frames. This does not always give smooth and natural movement of the moving body in the interpolated frame. By way of example, a spatially continuous line of the moving body may appear in the interpolated frame as a spatially discontinuous line at some of the picture blocks to result in a conspicuously degraded picture quality as will later be described with reference to one of eleven figures of the accompanying drawing.
A predictive encoding and decoding system is already known. On carrying out predictive encoding on a television signal comprising a movement vector signal representative of movement vectors of the type described above, a predictive encoder produces a prediction error signal which corresponds to the movement vector signal, namely, which results when predictive encoding is carried out on the movement vector signal. It has now been found that the decoder according to this invention is preferably a predictive decoder.